This invention relates generally to carbonaceous materials that have enhanced properties. More particularly, the present invention is related to carbon material that is made oxidation resistant to temperatures of 900° C. The oxidation resistant carbon materials have an electrically non-conducting surface with significantly enhanced surface hardeness.
Carbonaceous materials, such as carbon, graphite, carbon-carbon composites, glassy carbon, and the like have many uses. In particular they are useful at high-temperatures where they have excellent mechanical strength. The oxidation of carbonaceous materials in air or oxygen-containing environments at temperatures of 400 to 500° C. has limited its use in high-temperature applications. Otherwise, the easy machinability, low density, good strength, and other properties would lead to carbonaceous materials being the obvious choice.
Oxidation protection of carbonaceous materials has been directed to coatings and layers that are utilized to reduce the reaction of oxygen with the materials. Exemplary teachings are provided in U.S. Pat. Nos. 4,711,666 and 4,769,074. Often such layers contain silicon or aluminum to help form glasslike coatings during oxygen attack, whereby the glassy layer or glaze will reduce any additional oxidation of the substrate. An inherent concern with coatings is the thermal expansion mismatches between the substrate and coating that often cause delamination and complete coating spallation.
Another example of oxidation improvement for carbonaceous materials is U.S. Pat. No. 5,368,938, herein described is the reaction of carbon with gaseous boron oxide to form boron carbide. Still another method of oxidation protection for carbonaceous materials, described in U.S. Pat. No. 5,356,727, is based on “boron carbonitride” designated as CBN, or CBNO if it contains oxygen. CBN is produced by chemical vapor deposition at 700° C. with a mixture of hydrocarbons, boron trichloride and ammonia along with nitrogen or hydrogen carriers at a low as a small fraction of atmospheric pressure, such as a few hundred to a few thousand pascals. The CBN, as described therein, typically has a “metallic appearance” at 50 micrometers thickness.
Graphite has been coated with “pyrolytic boron nitride” to form boats for metal vaporization, as described in U.S. Pat. No. 4,264,803. In such cases, the boron nitride coating was deposited at 1750 to 2300° C. to a thickness of about 250 micrometers or 0.010 inches. It was found that the geometry of the boat cavity and nearly total encapsulation of the boat held the coating onto the substrate. The tendency of the coating of “pyrolytic boron nitride” to delaminate seems to be the main problem with this type of boat.
None of the known technologies for improving the oxidation resistance of carbonaceous materials produces a carbon material that is not a coated surface. Integral materials have been heretofore been thought to be difficult to prepare due to the differences in crystal lattice between dissimilar materials. Any blending of materials would generate a unique crystalline lattice which is dissimilar from either starting material. This typically leads to crystallographic defects and dislocations which can create additional, often uncontrollable and unpredictable, crystallographic phases.